The Long Term Evolution (LTE) standard defines a interface, called the X2 interface, for communication between eNodeBs. The X2 interface supports user plane communications in the form of the X2-UP protocol, which is used to tunnel end-user packets between eNodeBs via GTP-U over IP or UDP as the transport layer protocol. The X2 interface also supports control plane communications via the X2-CP protocol, which is used to provide various functions over SCTP as the transport layer protocol. The X2-CP application layer protocol is X2AP.
The X2AP protocol provides the following functions, as described in 3GPP TS 36.423, hereby incorporated in its entirety: mobility management; load management; reporting of general error situations; resetting an X2 connection; setting up an X2 connection; eNodeB (eNodeB) configuration update; mobility parameter management; mobility robustness optimization; energy saving; X2 release; message transfer; registration; and removing an X2 connection. For example, X2AP can be used to facilitate user equipment (UE)-assisted network-controlled handovers and handovers without the involvement of a mobility management entity (MME).
The X2 interface generally defines a mechanism via which eNodeBs can directly interact with each other in order to perform functions like UE mobility management and load management effectively without involving the evolved packet core (EPC). The present disclosure describes how multiple internal eNodeBs managed within a virtual eNodeB may communicate with each other, and with eNodeBs outside of the virtual eNodeB, using the X2 interface.
An X2AP Message Transfer message is known in the art as follows. The purpose of the X2AP Message Transfer procedure is to allow indirect transport of an X2AP message (except the X2AP MESSAGE TRANSFER message itself) between two eNBs and to allow an eNB to perform registration. The X2AP Message Transfer message includes an X2 Message IE that can include any X2AP message except the X2AP MESSAGE TRANSFER message itself. Typically, a first eNodeB eNB1 initiates the procedure by sending the X2AP MESSAGE TRANSFER message to eNB2. Upon the reception of X2 MESSAGE TRANSFER message the target eNB may: retrieve the X2AP message included in the X2AP Message IE; consider the target eNB ID contained in the Target eNB ID IE, included in the RNL Header IE, as the destination for the X2AP message signaled in the X2AP Message IE; and then consider the source eNB ID contained in the Source eNB ID IE, included in the RNL Header IE, as the source of the X2AP message signaled in the X2AP Message IE. In case the included RNL Header IE does not contain the Target eNB ID IE, the receiving eNB shall consider the eNB ID included in the Source eNB ID IE as the eNB ID corresponding to the TNL address(es) of the sender and update its internal information. X2AP Message Transfer is supported by Rel. 12 and following versions of the 3GPP LTE specification.